I work for a department store and the winter coats are already on the salesfloor. IMO, September or October would be when I would list - depends on the climate. Some areas get cold sooner than others, so that that in to consideration.
FWIW - I listed a sweater last night and woke up this morning to 2 bids, nearly doubling my starting bid of 2.99, and it's only mid-August.